Alkaline Process and System for Producing Pulp

ABSTRACT

A process and an apparatus are provided for producing pulp from lignocellulosic material by an alkaline cooking process, in which process preheated chips are treated in an impregnation stage with impregnation liquor, which is taken from the impregnation stage, and concentrated to obtain both a high inorganic and organic dry solid content. The temperature is higher than in the heating of the chips and in the removal of air, and is sufficient to enable the finishing of the reactions in the impregnation stage before the cooking stage. The majority of the alkali is added to the impregnation stage. In the cooking stage alkali is added to enable the delignification to be performed to a desired degree, and at the end of the cook black liquor is added to regulate the liquid-to-wood ratio. The process can be applied both to a continuous cook and to a batch cook.

FIELD OF THE INVENTION

The invention relates to an alkaline process for preparing pulpaccording to the preamble of claim 1 as well as a system for carryingout the process.

BACKGROUND OF THE INVENTION

The objective of chemical pulping is to remove lignin so that the fibrescan be separated with minor mechanical work. Among the chemical pulpingmethods, alkaline cooking processes and especially kraft or sulfatecooking are dominant in the production of chemical cellulose pulpbecause they provide pulp fibers which are stronger than those obtainedfrom other commercial pulping process. The lignocellulosic material,typically cut into wood chips, is treated in either batch or continuousdigesters with cooking liquor.

The active components of an aqueous solution of sulfate cooking liquorare hydroxide (OH) and hydrogen sulfide ions (HS). The delignificationtakes place mainly by action of OH ions, but also the HS content indecisively significant in two ways: the hydrogen sulfide ions protectthe carbohydrates of wood material whereby carbohydrate yield isimproved, and on the other hand, they accelerate delignificationreactions. Like the reactions of OH ions, also the rate of thesereactions is increased with increased temperature.

In practice, it is not possible to cook high-quality cellulose usingmerely hydroxide, but catalytically acting anthraquinone (AQ) isadditionally to be used. AQ may also be used in an ordinary kraftcooking, generally resulting in better yield and/or faster cooking. Theyield can also be improved by a polysulfide treatment which has notshown to be easy to carry out in practice in industrial processes. Theuse of anthraquinone is associated with problems, as well: The use of AQinvolves additional costs, and on the other hand, it cannot be used inall cellulose applications because of its toxicity.

Pores inside fresh wood chips are partly filled with liquid and partlywith a gas mixture consisting mainly of air. The ratio is, among others,determined by the density, moisture and dry content of wood. The airshould be removed from the chips before they can be perfectlyimpregnated with cooking liquor. This is usually done by treating thechips with steam, and with respect of the present invention, forexample, by using a process and a apparatus according to Finnish patentapplication FI 20021208.

There are two major stages where the transport of chemicals into thechips can occur: (1) the penetration in the impregnation stage where thechips are moistened with chemical-containing liquid beforedelignification reactions begin, and (2) in the continuous movement ofchemicals to the reaction sites during the cooking stage. Thepenetration of the steamed chips with the impregnation liquor takesplace fast. After that, reactive ions must diffuse into the chips. Therate of diffusion is most significantly influenced by differences in theconcentration of chemicals in the liquid outside the chips and in thepenetrated liquid inside the chips. In addition, the so-called Donnaneffect is to be taken into account which means that there must be acertain external ion concentration (generally given as Na concentration)in order to make it generally possible for the ions to be transportedinto chips.

In cooking there is a critical balance between the rate of iontransportation, wood porosity, chip dimensions and the rate of chemicalreaction. For instance raising the temperature increases the rate oftransportation, but also the rate of reaction increases. Chip dimensionsare of major importance in this context. The longer, wider andespecially the thicker the chips are, the longer is the transportationdistance to the central parts of the chips. If the transportationdistance is too long and the rate of transportation too slow, thechemicals may be completely consumed before the cooking liquor can reachthe chip centers, resulting in non-uniform cooking.

The higher the cooking temperature, the shorter time is required time toreach a certain delignification degree. A commonly used measure for thetime and temperature required for cooking reactions is the so-calledVroom H-factor expressing time integrate of the reaction rate as afunction of time. In order to reach a certain delignification degree, acertain chip raw material can be understood to require, under constantconditions, an approximately constant H factor value. Thus, e.g. ahigher cooking temperature provides a required H factor value in ashorter time.

A low cooking temperature is an objective strived for, besides for thereasons of energy economy, also because of the yield and uniformity ofthe cook, but on the other hand, it means slower cooking reactions andthus a longer reaction time to obtain a certain H factor and a certaindelignification degree. It leads naturally to larger digester volume,which, besides increasing investment costs, may at higher productionlevels cause problems in runnability of the process and thus in pulpquality.

The delignification reactions are generally divided to take place inthree different steps: extraction delignification (1), bulkdelignification (2) and residual delignification (3). In practice, themajority of the delignification reactions take place in step (2), andthe reactions of step 3 are attempted to be avoided, because in thatstep cooking selectivity is essentially poorer than in step 2. Thedelignification occurring during the step 1 is not selective, either,due to the fact that in this step many various reactions with chemicalcompounds of wood material take place. Extraction delignification can besaid to be based on the extraction of lignin bound to variouscarbohydrates, such as hemicellulose, from wood material. This, however,requires reactions with carbohydrates and thus a decrease incarbohydrate yield.

In existing continuous processes, typically referred to as the Kamyrtype, chips are heated with steam and air is removed from the chips tofacilitate later liquor impregnation. In continuous cooking, the chipimpregnation stage typically takes place, in various modifications in aretention time of 10 to 60 minutes at a temperature of 100 to 145° C.Since penetration rates increase with increased pressure, impregnationstages typically operate at operating pressures of about 5 to 10 bars atthe aforesaid temperatures. Subsequent to impregnation, the chips areheated directly in a vapour phase and/or in several liquor heatingcircuits to a cooking temperature, and then typically cooked for atleast 90 to 150 minutes in a concurrent and/or in a countercurrentcooking zone at temperatures below 165° C., in some overstresseddigesters the maximum temperature may be even higher than that.Practical experience shows that the process becomes limited by chemicaldiffusion at cooking times of below about 90 to 150 minutes andtemperature above 165° C.

Typical cooking temperatures for softwood material are 145-165° C.,whereas for hardwood lower temperatures of 135-150° C. can be used, cf.e.g. international patent application WO 98/35091. Thus, a cooking timeof about 90-150 minutes is required. In addition, subsequent to theconcurrent cooking zone, there is usually a concurrent or countercurrentzone of 60 to 240 minutes at temperatures of 130-160° C. Contemporarycontinuous cooking processes as e.g. ITC, EMCC and Lo-solids cookingtypically retain the cooking temperature through almost all of theaforesaid cooking zones, i.e. utilizing nearly the whole volume of thepressure vessel for cooking. These modern digesters have thus a totalcooking zone of about 180-360 minutes. For the countercurrent zoneand/or for cooling the blow pulp, washing filtrate is pumped into thebottom of the vessel. A blow temperature is typically 85-95° C.

Several methods have been described to increase the HS ion concentrationespecially in the impregnation stage. Both in batch and continuousprocesses, these methods are based on the utilization of the blackliquor from the cooking stage in the impregnation, e.g. U.S. Pat. Nos.5,053,108 and 5,236,553. In practice, this means, however, an increasein the relative HS ion content compared to the OH ion content sincesignificantly more OH ions than HS ions are consumed by thedelignification reactions. However, a high HS-to-OH ratio does notnecessarily mean that the absolute HS content would be very highcompared e.g. to white liquor. Thus, the content difference being adriving force for diffusion, the efficiency of the impregnation is notnecessarily efficient as to the HS ions.

The concentrations of the cooking liquor (dissolved dry matter, OH andHS ions) in a certain cooking stage are determined by the requiredamount of alkali, i.e. alkali consumption, the moisture of the chips andthe dilution introduced with chemicals. The HS ion content in whiteliquor, being essential for the rate of the delignification reactions,is in fact high, but it cannot be added to exceed the amount ofconsumption in the cooking stage, resulting in spent liquor stillcontaining a large amount of alkali, thus impacting the recovery ofchemicals. In addition, a too high OH concentration affects negativelythe yield of the cooking. On the other hand, the cooking liquor isconsiderably diluted by chip water. For this reason, it would beadvantageous that the chips are as dry as possibly when entering theprocess. This is, however, not reasonable with respect to the qualitynor the operability of the process, and it is not possible, either, todry chips energy efficiently under control.

Patent application WO 03/062524 describes a method in which black liquortaken from cooking is evaporated before it is returned to the beginningof the cooking stage. From the end of the cooking stage the black liquoris intended to be fed to impregnation. The aim of this is to obtain ahigher dry content which should improve cooking yield. The arrangementof this kind would also increase the absolute concentration of HS ionsin the cooking stage. However, in several studies, it has been foundthat HS ions should be present already at the beginning of the wholeextraction delignification and the bulk delignification to fully utilizethe yield advantage and the increased rate of the cooking reactionsobtained by them.

Swedish patent SE 521678 describes a method to increase sulfideconcentration in impregnation in which method black liquour is takenfrom two different stages of the cooking and from which the latter canbe introduced to the impregnation through a expansion tank. Theindependent claim states that the cooking temperature is in the range of150-180° C., and in connection with sulfide treatment the temperatureshould be at least 10° C. lower than the temperature of the cookingstage. The method enables to increase the HS concentration to someextent in the impregnation and in the initial stage of the cooking, butthe black liquor from the cooking stage cannot have a very high HSconcentration without adding a relative great amount of alkali to thecooking stage since the majority of the consumption takes place in theimpregnation stage. This would, in turn, lead to essentially decreasedyield.

DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide a process and asystem which enable the use a temperature as high as possible in theimpregnation of an alkaline cooking process so that carbohydrate yieldafter both the impregnation and the cooking stage can be maximized.

A process according to the invention is characterized by what isdisclosed in the characterizing part of claim 1. A system according tothe invention is characterized by features disclosed in thecharacterizing part of claim 44.

The process according to the invention provides impregnation conditionsenabling high concentrations of dissolved organic and inorganic drymatter of the impregnation liquor compared to those in the prior art. Inthe cooking process according to the invention, the extractiondelignification is carried out under optimal conditions as far aspossible so that the extraction delignification would occur as little aspossible or not at all in connection of the bulk delignification.

The system according to the invention enables to carry out the alkalinecooking according to the invention so that the liquid circulations ofthe impregnation stage and the cooking stages are separated from eachother, and the impregnation liquor circulating in the impregnation stagecan be concentrated.

When proceeding according to the invention, carbohydrate yield after alldelignification stages is maximized. In addition, due to the extractiondelignification to be carried out in the impregnation stage and due tothe efficient sulfide and/or AQ treatment, the bulk delignification tobe carried out in the actual cooking stage can be carried out rapidly.Advantages achieved by the invention are i.a.:

-   -   possibility to use a lower cooking temperature whereby the        energy economy of the cooking process is improved and production        costs are reduced    -   possibility to increase the production capacity of a process        apparatus of a certain size    -   increased cooking yield results in lower consumption of raw        material and cooking chemicals    -   a lower cooking temperature enables the use of a low enthalpy        steam enabling higher production of electricity by means of a        counterpressure turbine    -   evaporation of the impregnation liquor reduces correspondingly        the evaporation required in an evaporation plant enabling        technically and economically improved evaporation arrangements.

The process according to the invention can be carried out in acontinuous cooking process in a one- or two-digester cooking process, ina steam/liquid phase digester or in a hydraulic digester. Impregnationis performed either in an upper part of the digester or before thecooking stage or in a separate impregnation vessel. The main principlesof the process according to the invention are applicable also to a batchcook especially a displacement batch cook. The raw material can inprinciple be any lignocellulosic material. Different raw materials suchas softwood, hardwood, bagasse and the like can be utilized in thisprocess. Likewise, various end products such as bleached or unbleachedpulp and a high-yield kraft pulp which optionally requires mechanicaldefiberation, can be produced by this process.

In an embodiment according to the invention, the black liquor from thecooking stage is not introduced to the impregnation in order to increasethe level of the HS and of dry matter level which is a significantdifference compared to prior art embodiments which utilize the HS/OH ionratio of the black liquor generated in the cooking stage. Liquid istransported to the cooking from the impregnation stage only inside thechips and in an amount carried during the transport of the chips. Whenthe liquid-to-wood ratios of the impregnation stage and the cookingstage are separated from each other, the possibility to regulate theprocess in different stages is improved. In addition, the liquids differfrom each other as to their compositions and they contain reactionproducts from corresponding stages and cooking chemicals required for astage. By the conditions in the impregnation, such conditions of theextraction delignification are maintained inside the chips which enablethe reactions of the lignocellulosic material and the HS ions and/or theAQ reactions to occur as efficiently as possibly, i.e. a high HS ionconcentration and/or AQ concentration in free liquid and a temperatureas high as possible.

DETAILED DESCRIPTION

In this description by impregnation liquor (IL) is meant an alkalineprocess liquor used in the impregnation stage.

By cooking liquor (CL) is meant a liquor to be used in the bulkdelignification, which liquor is removed from the digester or blown offtogether with pulp after the end of the cooking.

The terms white liquor (WL), black liquor (BL) and green liquor havetheir common meanings used in sulfate processes.

By fresh alkali is meant an alkaline solution from the preparing systemof chemicals, which in sulfate processes is white liquor or white liquorand green liquor. In soda processes the fresh alkali is mainly NaOH. Insoda processes the fresh alkali corresponds to white liquor but it doesnot contain substantially sulfide. Fresh alkali does not substantiallycontain dissolved organic dry matter.

Essential in the concept according to the invention is that free wateris remover from the impregnation in stem form, e.g. by evaporation or byexpansion, whereby high concentration differences required for diffusionmaintain high. In one special embodiment the removal of water iseffected partly or wholly in liquid form, e.g. by a membrane film. Thefunction of cations of the inorganic dry solid matter present in theimpregnation liquor is to counteract the aforesaid Donnan effect, andthe function of the organic matter is to buffer the impregnation liquoragainst yield losses of the chip material. By retaining in theimpregnation carbohydrates dissolved in the hydrolyse occurring at thebeginning of the impregnation, improved yield is obtained andadditionally the carbohydrates have a catalyzing effect on the cooking.The level of the inorganic and organic dry solid matter dissolved in theimpregnation liquor is between 15-50%, preferably 20-35%.

This enables the use of a higher impregnation temperature withoutnegative effects on yield, on the conformity of the cooking or on thepaper technical properties of the pulp. Due to the high temperature,hydrogen sulfide reacts with lignin and is attached chemically to woodmatrix. The sorption of hydrogen sulfide and the reactions with ligninare the primary objectives of the high impregnation temperature. Thisleads to a higher rate of the bulk delignification and to an increase incarbohydrate yield. If the alkali added to the process does not containsulfide, or it is desired to make the sulfate process more effective,anthraquinone (AQ) or a corresponding additive is added. Theaforementioned, with respect to hydrogen sulfide advantageous conditionscan preferably be utilized in the use of AQ.

The impregnation takes place by means of an alkaline impregnation liquorcirculating in the impregnation stage to which fresh alkali is fed as analkali addition and wherein the impregnation liquor is concentrated byremoving water, e.g. by evaporating or by flashing, from it. The processaccording to the invention is especially preferably sulfate cooking andthe fresh alkali consists preferably essentially or entirely of whiteliquor. In an embodiment alkali addition consists merely of fresh alkalior essentially merely of fresh alkali. In a preferred embodiment thealkali addition consists of fresh alkali and black liquor which is fedup to 1.0 m³/Odt. The cooking process may also be soda cooking, wherebythe fresh alkali consists of sodium hydroxide as dry or as a solution.In an embodiment, also anthraquinone is added to the impregnation stageand/or the cooking stage. In an embodiment the fresh alkali addition fedto the impregnation liquor may consist of green liquor or green liquorand another fresh alkali, particularly white liquor.

In the impregnation, the temperature is maintained as high as possiblealso in order to allow necessary degradations of the carbohydratesrapidly consume the hydroxide of the white liquor which will increasethe HS-to-OH ratio. In addition, the removal of water increases thehydrogen sulfide concentration to a high level. The upper limit of thetemperature is determined by the uniformity of the impregnation, theconsumption of alkali and balances of the alkali as well as the startingof the bulk delignification. Thus, the temperature of the extractiondelignification in the impregnation stage is 135±55° C., preferably135±20° C., strongly depending on the raw material. In one embodimentthe temperature is 135±35° C., preferably 135±10° C. It is alsoessential to carry out the extraction delignification stage as close toan end as possible so that this stage would not be carried outsimultaneously with the bulk delignification or that extractiondelignification would further occur as little as possible during bulkdelignification. The target yield by the impregnation is 80±15%,preferably 80±10%, depending on the raw material to be used. In this thetotal yield after all delignification steps is maximized.

The impregnation may comprise several steps and consists preferably ofone or two steps. In the first step, a rise of the ratio of HS ions toOH ions is achieved towards the end of the step since the hydroxide israpidly consumed at the beginning of the impregnation. In this way by acertain alkali amount it is possible to provide more reactive HS ionsinside the chips. The objective of the second step is to rapidlyincrease the alkali concentration before the bulk delignification whichpromotes uniform impregnation of the effective alkali to a chip,resulting in uniform cooking. Essential in the impregnation is a highdynamic liquid-to-wood ratio, i.e. between the chips and the liquidphase there is a two to sevenfold, preferably a three to fivefold flowrate difference. In a preferred embodiment of the process according tothe invention the extraction delignification is carried out ascompletely as possibly during the impregnation, particularly in thefirst step thereof. In a continuous process, the first step of theimpregnation is carried out in a concurrent flow, and the second step ofthe impregnation may be concurrent or countercurrent. In a preferredembodiment the impregnation stage is merely concurrent. Yet in anotherembodiment the impregnation stage is merely countercurrent. At thebeginning of the impregnation steps the amount of the effective alkalidoes not exceed 1.5 mol/l NaOH, is preferably 0.5-1.5 mol/l NaOHparticularly 0.8-1.2 mol/l NaOH, preferably neither during theimpregnation steps. In an embodiment, a rapid rise of the alkaliconcentration in the second step of the impregnation may be carried outat the beginning of the cooking stage before the bulk delignificationbegins, when the temperature of the cooking stage is low, preferably140±10° C. The amount of the alkali required for the bulkdelignification is small compared to the total consumption, particularlywhen using hardwood. In a preferred embodiment, due to the effectiveimpregnation and hydrogen sulfide treatment, after the temperature risein the cooking stage the process proceeds directly to bulkdelignification which bulk delignification is rapid. In a preferredembodiment the impregnation comprises a first step and a second step,whereby in the second step the OH concentration and the HS concentrationare essentially higher than the corresponding concentrations at the endof the first step. Yet in another embodiment the HS concentration in thesecond step of the impregnation is essentially higher than at the end ofthe first step. Yet in another embodiment the OH concentration in thesecond step is essentially higher than at the end of the first step.

The alkaline cooking process according to the present invention forpreparing pulp from lignocellulosic raw material comprises theimpregnation of the raw material and the cooking of the impregnated rawmaterial with cooking liquor. In an embodiment the raw material issteamed before its impregnation. In an embodiment of the process theconcentration of the impregnation liquour is effected by heat energyfrom the cooking stage. In an embodiment the concentration of theimpregnation liquor is effected partly by means of heat energy broughtfrom the cooking stage, whereby additionally other energy is used. In anembodiment other energy than secondary energy from the cooking stage isused, e.g. low pressure steam or electric energy, e.g. by means of acompressor. In an embodiment water is evaporated from the impregnationliquor by heating the impregnation liquor first indirectly by steam andby evaporating, and after that, the impregnation liquor is heatedindirectly by means of the black liquor from the cooking stage to animpregnation temperature.

In an embodiment of the process according to the invention no energy isintroduced to the process from the outside after the impregnation stageto elevate the temperature.

In an embodiment of the process according to the invention the majorityof the alkali to be added to the raw material is added preferably to thesteps of the impregnation.

The cooking stage of the process according to the invention maypreferably comprise both a concurrent and a countercurrent zone, whosealkali concentrations and temperatures can be regulated.

The cooking reactions of the process according to the invention can beterminated in the digester e.g. by displacing the cooking liquor or bydiluting using filtrate from a wash plant, whereby the blow is carriedout at a temperature below 100° C. In a further embodiment the pulp isblown out essentially at a temperature higher than 100° C., optionallyinto a blow tank provided with heat recovery. In a further embodimentthe pulp is blown out through a mechanical defibering stage into a blowtank.

According to laboratory test results, the arrangement according to theinvention provides improved pulp yield due to the effective HStreatment. Likewise, the rate of the delignification increasessignificantly: according to the test results the H factor demand is evenhalved. Bleachability of the pulp is good due to the effective HStreatment.

EXAMPLE 1

Pulp was cooked under prior art conditions from eucalyptus chips in alaboratory, whereby the temperature in the first step of theimpregnation was 110° C. (duration 45 min) and in the second step 120°C. (15 min). The cooking temperature was kept at 152° C. for 120 minutesto obtain cellulose pulp having kappa number 17, which expresses thedegree of delignification of the cellulose pulp. The liquors used weretypical as to their HS and dry solid contents. The H factor expressingthe ratio of the temperature to the cooking time was about 460. Pulpyield from the chips was 53.7% on the wood amount used.

EXAMPLE 2

Using the same material, by circulating and by concentrating theimpregnation liquor during several cookings, such conditions werecreated which correspond to those according to the invention: thedurations of the impregnation as in Example 1, the impregnationtemperature of 130° C. and the dissolved dry solid content and the HSconcentration were approximately double compared to those of the cookingof example 1. Nearly the same degree of delignification (kappa number18) was obtained at a temperature of 149° C. in a time of 100 minuteswhich means a H factor of 230 which is half of that of example 1.Cellulose pulp yield was 54.5% on the wood amount used.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a preferred embodiment of the invention schematically.

FIG. 2 shows a preferred embodiment of the invention with a separateimpregnation vessel.

FIG. 3 shows a preferred embodiment of the invention where impregnationand cooking are performed in the same vessel.

DETAILED DESCRIPTION OF THE PROCESS ARRANGEMENT

The arrangement according to the invention can be carried out e.g.according to FIG. 1, wherein in chip treatment (1) the chips are treatedwith steam, which may be fresh or generated by flashing the liquor, toheat the chips and to remove air (gas mixture) from the chips. Afterthat, a chips-liquor mixture with impregnation liquor is formed whichmixture is introduced to impregnation (2). The impregnation (2)comprises one, two or more steps having different chemical and/ortemperature profiles. The first step is carried out preferably in aconcurrent flow, and the second step, depending on the case, in aconcurrent or countercurrent flow. The duration of the first step of theimpregnation (2) is 15-120 minutes, preferably 15-45 minutes, and thatof the second step of the impregnation is 5-60 minutes, preferably 10-45minutes.

Liquor (IL) can be removed at the beginning of the impregnation steps(2), between the impregnation steps or before the cooking stage (3) e.g.by means of a screen zone or a screen/screw arrangement. To this removedliquor (IL) fresh alkali (white liquor) can be added, it can be heateddirectly or indirectly (8) and be introduced to expansion and/orevaporation (9). After the expansion/evaporation the cooled liquor (IL)can be introduced into a level tank (10) for stabilizing flowvariations. Separation of extractives and soap can also be carried outin the tank. Too much alkali (white liquor) should not be added to theliquors (IL) of the impregnation stage to avoid additional yield losses.The hydroxide peak caused by the white liquor addition is diluted by ahigh dynamic liquid-to-wood ratio, i.e. by circulating free liquid. Thisdynamic liquid-to-wood ratio may be 2-10 m³/Odt wood, preferably 3-6m³/Odt. In an embodiment of the process according to the invention theblack liquor (e.g. from the cooking stage) may be added to theimpregnation liquor, e.g. reject from fiber separation, 0-1.0 m³/Odtwood, preferably below 0.5 m³/Odt.

The removal of water (here evaporation) contained in the liquors (IL) ofthe impregnation stage (2) may be carried out e.g. in an expansion tankand/or in an evaporation unit (9), in one or more steps. Subsequent tothat, the concentrated liquor flow may be heated to a desiredtemperature in one or more heat exchangers (11) wherein the heating flowmay be e.g. steam, black liquor from the expansion screens of thedigester or another secondary energy generated in the process or acombination thereof. This liquor (IL) to be circulated is returned tothe impregnation stage at one or more points.

From the impregnation stage (2) some or no free impregnation liquor istransferred to the cooking stage (3) of the cooking process, to the bulkdelignification stage of the cooking. To the cooking stage (3), cookingliquor CL may be introduced which may consist of black liquor circulatedfrom the end of the cooking and fresh alkali, here white liquor,optionally added thereto. The amount of the circulated black liquordepends on a desired dynamic liquid-to-wood ratio in the cooking stage(3). The dynamic liquid-to-wood ratio in the cooking stage (3) may bee.g. 2-6 m³/ODt (oven dry), preferably 3-4 m³/ODt. If the liquid in theliquid to wood ratio is considered to be free water only, thecorresponding values are 1-5 m³/ODt, preferably 2-3 m³/ODt. In thecooking stage (3), the temperature is maintained at 150±20° C. for asufficient time to obtain the required H factor. The black liquorremaining at the end of the cooking stage (3) is first cooled by one ormore heat exchangers (11), wherein the impregnation liquor is intendedto be heated, and after that by one or more heat exchangers using coldwater, if required. The cooled black liquor is introduced to recovery(5-7) of cooking chemicals. After the cooking stage (3) the pulp iscooled and transferred to washing (4).

The recovery (5-7) of cooking chemicals consists of evaporation unitswherein black liquor (BL) is evaporated to raise its dry solid contentto a level of 65-85%. In practice, in the embodiment according to theinvention, a part of the evaporation work is carried out preferably in awater removal unit (9) in connection with the impregnation (2), wherebya number of the evaporation units (5) needed for the concentration ofthe black liquor can be correspondingly smaller. Since the dry solidlevel of the feed liquor (BL) from the cooking stage (3) is higher thanthat in a cooking plant, where the evaporation is not carried out inconnection with the impregnation (2), so-called feed liquorstrengthening is not necessarily required in the evaporation plant (5),resulting in an increase of the capacity of the evaporation plant (5)and in improved energy economy. In spite of that, the water removal unit(9), e.g. an evaporation unit, can be placed at the recovery (5-7), e.g.in a process modification to the embodiment according to the invention.In this case, however, the flow to be evaporated (IL) is still separatedfrom other evaporation units (5) which evaporate black liquor (BL). As alast step, a superconcentrator (6) may be used before the black liquoris burnt in a soda recovery boiler (7). In this connection, a melt isformed which is dissolved to green liquor to obtain white liquor bycausticizing (not shown in the figure).

The process according to the invention can be carried out in acontinuous process or batch cooking process, especially in adisplacement batch cook. A preferred embodiment of the invention in abatch cook comprises a step before the impregnation stage in which chipsare treated with steam to heat the chips and to remove gases. In thebatch cook a separate impregnation vessel may be used from which achips-liquor mixture is filled to digesters. The process may, inprinciple, be performed in present apparatuses, by changing i.a. liquidcirculations so that the liquid circulations of the impregnation stageand the cooking stage are essentially separated from each other. Theimpregnation (2) may be carried out e.g. in a separate impregnationvessel or in the same vessel as the cooking stage (3), however prior tothe bulk delignification.

In the system according to FIG. 1 for carrying out the process accordingto the invention separated liquid circulations are provided for theimpregnation liquor (IL) and the cooking liquor (CL) and they areessentially separated from each other.

In FIG. 2 is shown one preferred embodiment of the invention with aseparate impregnation vessel. By letter P before a reference number inmeant a pump. The chipped fiber material is fed through a chip bin to achip-treatment vessel (20), where air is removed from the chips. Thesaid vessel (20) can be e.g. according to a heating, steaming andstorage apparatus shown in the patent application FI 20040637. Afterthis the fiber material is slurried by means of a liquid flow (50) whichin this case is impregnation liquor and the slurried fiber material (51)is fed to a chip feeder (21). In the embodiment of FIG. 2, the chipfeeder is a feeder with which material in slurry form can by transportedfrom a first pressure system to a system having a second, higherpressure.

In the embodiment of FIG. 2 the impregnation is with two steps: beforescreens (S1, S2) the impregnation is concurrent and after the screenscountercurrent. The slurried fiber material is fed by means of the chipfeeder (21) along line 52 to a continuous impregnation vessel (22).Impregnation liquor used for the transportation of the chips isseparated from the chips in the upper part of the impregnation vessel bymeans of equipment known in the art. Transportation liquid (53) isreturned by means of the chip feeder apparatus (21) to the evaporationcirculation where impregnation liquor is concentrated. The impregnationliquor (54) leaving the chip feeder in pumped (P3) to a pressure screen(23) in order to remove fiber material, whereby the reject flow (55)obtained is guided back to the slurrying of chips. The pressure screen(23) can alternatively be replaced with a tube screen known in the art.Fresh alkali (here white liquor WL) is fed to the accept flow (56) fromline 57. The impregnation liquor is heated in a heat exchanger (24)whereto low pressure steam (57) is fed.

The heated liquor flow is directed to a water removal unit 25 along line84. In the water removal unit 25 the impregnation liquor is concentratedby removing water from it. Part of the steam (58) exiting the waterremoval unit 25 is directed along lines 59, 60 and 61 to be used in chipheating and pre-steaming, and part of it is removed wholly from theimpregnation circulation and cooking circulation and is directed to thecondenser. Depending on the amount of chip water and on theconcentration of the fresh alkali (57) added, the impregnation liquormay be concentrated further at the second water removal unit 26 and thesteam generated is directed to condenser. Water removal units 25 and 26can be e.g. expansion tanks and/or evaporation units. The impregnationliquor concentrated at water removal unit 25 is directed along line 85to a water removal unit 26.

The concentrated impregnation liquor (62) is fed to a level tank (27).In the embodiment according to FIG. 2, soap is removed from theimpregnation liquor in the level tank, after which impregnation liquor(63) is pumped (P5) to an impregnation vessel and if necessary, alongline 50 for slurrying of the chips. The surface level of the level tankis regulated by the magnitude of flow 63. The impregnation liquor (63)fed to the impregnation vessel is heated in a heat exchanger (28)utilizing the heat of the black liquor (64) coming from the digester.

The heated liquor flow (65) is pumped (P4) through a heat exchanger (29)to the bottom of the impregnation vessel (22). Before the heat exchanger(29) and the pump (P4) the impregnation liquor is combined with a liquorflow from the upper part of the digester, which liquor has been used totransport the impregnated fiber material from the impregnation vessel tothe digester (30). In the said heat exchanger (29) low pressure steam(66) directed thereto is used.

The concentration of the chip-liquor-mixture, here moving from theimpregnation vessel to the digester, is regulated with pump P4.Impregnation liquor is removed from the impregnation vessel thoughscreen S2 to line 68 and through a central tube screen along line 69which joins line 68. Impregnation liquor that has passed through asecond impregnation step, is pumped (P7) along line 68 to theimpregnation liquor-chip-flow 52 going to the impregnation vessel,whereby this impregnation liquor is returned to the water removal of theimpregnation liquor. From the first step of the impregnation, theimpregnation liquor which is diluted by the water in chips, is removedthrough screen S1 and through the central tube screen and through line70 to line 71 wherefrom it is pumped to return circulation 53 andthereby returned to the water removal of the impregnation liquor. Partof the flow 71 is directed directly to a heat exchanger 24 preceding thewater removal unit 25 by means of line 72. After the heat exchanger (28)of line 65, starting from the level tank (27) the impregnation liquor isdirected through line 65′ to the chip-impregnation liquor-flow (52)going to the first step of the impregnation vessel. Steam generated atthe first water removal unit 25 can be directed to chip steaming in thechip treatment vessel (20) and/or to the chip bin preceding it (not inthe figure). The impregnation liquor which is used in impregnation andis circulating in the impregnation, is concentrated when part of thesteam generated in the water removal unit 25 and the steam (14)generated in the water removal unit 26 in its entirety, is removed fromthe whole system to the condenser, i.e. part of the steam is notreturned to the chips. Steam and air (83) from the chip treatment vessel(20) is removed to the condenser.

The impregnated fiber material is removed from the bottom of theimpregnation vessel (22) and is transferred to a continuous digester(30) along line 73. The embodiment according to FIG. 2 is a steam-liquidphase-digester. Impregnation liquor used for transportation of chips isremoved from the top of the digester by means of equipment known in theprior art and the said impregnation liquor is returned along line 74 bymeans of pump P4, through a heat exchanger 29 to the bottom of theimpregnation vessel (22).

The digester (30) is heated with low pressure steam or with middlepressure steam (81) which preferably originate from the power plant ofthe mill. The dynamic liquid to wood-ratio in the upper part of thedigester is regulated by pumping with pump P11 black liquor sucked fromthe central tube screen along line 75 to the upper part of the digester.Fresh alkali (here white liquor WL) is added to line 75 in order toregulate alkali. Black liquor removed by central tube screens is addedthrough line 76 to the black liquor flow leaving the digester throughexpansion screens S3 and S4.

Black liquor removed from the digester (30) through screens (S3, S4) andthrough central tube screens is used to heat at the heat exchanger (28)the impregnation liquor from the level tank to the impregnation vessel,after which the black liquor is directed to a pressure screen 31. Theaccept stream of the pressure screen is directed to the chemicalrecovery and the reject (77) to washing filtrate (78) or to the stream(50) for slurrying the chips. From the screen S5 located in the lowerpart of the digester (30) black liquor is removed through stream 79 andis circulated to screen S5. White liquor (80) is added to this stream(79). Washing filtrate (78) is fed to the bottom of the digester (30)with pump P13. In case when the step below screen S4 is countercurrent,only a small amount of black liquor is removed to line 64′ through thescreen S5. When the step is concurrent, the black liquor is removeralong line 64′. Stream 79 can be heated by means of heat exchanger 31,whereto low pressure steam (82) is fed.

In the system according to FIG. 2 for carrying out the process accordingto the invention separated liquid circulations are provided for theimpregnation liquor (IL) and the cooking liquor (CL) and they areessentially separated from each other.

FIG. 3 shows a preferred embodiment of the invention where impregnationand cooking are performed in the same vessel. Chips are fed to the chipbin (100) wherefrom the heated chips are fed by means of a low pressurefeeder (101) to a pre-steaming vessel (102) to remove air from thechips. Chips are fed from the pre-steaming vessel (102) through a chipchute (103) to a chip feeder (104) by means of which the chips are fedto a digester (105) with a higher pressure. The chip feeder (104) can bee.g. a high pressure feeder according to prior art. Impregnation andcooking of the chips is performed in the digester (105). In the figureletter P before a reference number refers to a pump. From thepre-steaming vessel (102) steam and air (108) can be removed anddirected to a condenser.

The slurried fiber material is fed by means of the chip feedingapparatus (104) along line 150 to the digester (105). Impregnationliquor used for the transportation of the chips is separated from thechips in the upper part of the digester by means of equipment known inthe art. Transportation liquid (151) is returned by means of the chipfeeder apparatus (104) to water removal where impregnation liquor isconcentrated. The impregnation liquor (152) exiting from the chip feeder(104) is pumped (P24) to a pressure screen (99) in order to remove fibermaterial, whereby the reject flow (182) obtained is guided back to thechip chute (103). The pressure screen (99) can alternatively be replacede.g. with a tube screen known in the prior art. The accept flow (153) isheated in a heat exchanger (106) whereto fresh steam (154) is fed.

The heated impregnation liquor (155) is directed to the first waterremoval unit 107, where the impregnation liquor is concentrated byremoving water from it. Part of the steam leaving from the water removalunit 107 is directed along line 156 to be used in chip pre-steaming(102) and the rest (157) is directed to a heat exchanger (109) to heatthe impregnation liquor. The impregnation liquor can be concentratedfurther at the second water removal unit (108) wherefrom the leavingsteam (158, 159) is directed to a condenser and to chip bin (100) forheating and steaming the chips. The water removal units (107, 108) canbe e.g. expansion tanks and/or evaporation units.

The concentrated impregnation liquor (160) is directed to a level tank(110). From the line (160) impregnation liquor is returned to the waterremoval through line 161. In the line 161 fresh alkali (here whiteliquor WL) is added from line 162 to the impregnation liquor and themixture is pumped with pump P25 to the heat exchanger 109 wherefrom itis directed to line 153.

In the level tank (110) soap is removed from the impregnation liquorwhich soap is removed along line 163 by means of pump P22. After soapseparation the impregnation liquor (164) is pumped with a pump P27) toimpregnation. Impregnation liquor 164 which is fed to the impregnationsteps is heated in a heat exchanger (111) utilizing the heat of theblack liquor (165) coming from the digester.

In the embodiment according to FIG. 3, the impregnation is in two steps:before the first screen (S21) impregnation us concurrent and between thefirst (S21) and the second screen (S22) concurrent or countercurrent.Impregnation liquor is removed from the first step of the impregnationthrough screen S21 to water removal (stream 178). The concentratedimpregnation liquor is returned to the first step of impregnation to theupper part of the digester (164) and to the second step of theimpregnation along line 166 either to heating circulation 167 and/or168. In case the second step of the impregnation is concurrent, line 168and pump P29 are used. Lines 167 and 168 include heat exchangers (notshown in the figure) working with fresh steam in order to heat theimpregnation liquor.

The cooking step subsequent to the second screen S22 is concurrent.Black liquor from the end of the cooking can be circulated from screenS23 by means of a pump P214 along line 169 to the beginning of thecooking, below the screen S22. The step between the screens S23 and S24can be concurrent or countercurrent. In the countercurrent step liquoris circulated along line 170 which is equipped with a heat exchanger, towhich line fresh alkali (here white liquor) is added (line 174) beforepump (P210). Black liquor is removed from the digester along line 165.In case when the step between the screens S23 and S24 is concurrent,cooking liquor is removed along line 170′. The heat of the black liquorremoved from the digester is utilized for heating of the white liquor inheat exchanger 11, after which the black liquor is directed to, viacooling and fiber separation (112, 113) to the chemical recovery alongline 172. In the cooling the pressure of the black liquor is decreasesin an expansion tank 114 and simultaneously odorous gases of the blackliquor can be removed which gases are not removed in the connection ofthe water removal (107, 108) because of high pH. Final cooling iseffected after the expansion tank 114 in heat exchanger 115, wheretofresh steam (179) is brought. In the fiber separation of the blackliquor coming from the digester, accept (172) of the first pressurescreen (112) is directed to the recovery of the cooking chemicals andreject (173) to the second pressure screen (113), the reject (174) ofwhich is directed to the washing filtrate (line 175) or to the line 182leading to the chip chute (103).

In the system according to FIG. 3 for carrying out the process accordingto the invention, separated liquid circulations are provided for theimpregnation liquor (IL) and the cooking liquor (CL) and they areessentially separated from each other.

1. An alkaline cooking process for producing pulp from lignocellulosicmaterial, said process comprising impregnation of the raw material andcooking of the impregnated raw material in a steam/liquid phase orhydraulic digester, wherein the impregnation is performed with analkaline impregnation liquor circulating in the impregnation stage, towhich liquor fresh alkali is introduced as an alkali addition and whichimpregnation liquor circulating in the impregnation stage isconcentrated by withdrawing impregnation liquid from the impregnationstage, removing water thereof and feeding the concentrated impregnationliquor back to the impregnation stage and that impregnation is performedat 135±35° C., the level of the inorganic and organic dry solid matterdissolved in the impregnation liquor is between 15 to 50%, and cookingis performed at 140±10° C.
 2. The process according to claim 1, whereinthe cooking process is a sulfate cook, and the fresh alkali consistsessentially or entirely of white liquor.
 3. The process according toclaim 1, wherein the alkali addition is merely fresh alkali oressentially merely of fresh alkali.
 4. The process according to claim 1,wherein the alkali addition consists of fresh alkali and of black liquorwhich is introduced at the most 1.0 m³/Odt.
 5. The process according toclaim 1, wherein the energy required for the evaporation to be carriedout to concentrate the impregnation liquor is taken partly or entirelyfrom the cooking stage.
 6. The process according to claim 1, whereinwater is evaporated from the impregnation liquor by heating theimpregnation liquor first indirectly with steam and by evaporatingimpregnation liquor.
 7. The process according to claim 1, wherein afterthe evaporation the impregnation liquor is heated by means of the blackliquor from the cooking stage.
 8. The process according to claim 1,wherein after the evaporation the impregnation liquor is introduced intoa level tank for stabilizing flow variations.
 9. The process accordingto claim 8, wherein the separation of extractives and soap is carriedout in said tank.
 10. A cooking process according to claim 1, whereinthe cooking process is a soda cook.
 11. The process according to claim1, wherein the impregnation comprises a first step and a second step.12. The process according to claim 11, wherein in the second step the OHconcentration and the HS concentration are essentially higher than thecorresponding concentrations at the end of the first step.
 13. Theprocess according to claim 11, wherein in the second step the HSconcentration is essentially higher than at the end of the first step.14. The process according to claim 11, wherein in the second step the OHconcentration is essentially higher than at the end of the first step.15. The process according to claim 11, wherein the second step of theimpregnation is carried out at the beginning of the cooking stage. 16.The process according to claim 11, wherein the impregnation comprisesseveral steps.
 17. The process according to claim 11, wherein theduration of the first impregnation step is 15-120 minutes.
 18. Theprocess according to claim 11, wherein the duration of the secondimpregnation step is 5-60 minutes.
 19. The process according to claim11, wherein the impregnation stage is merely concurrent.
 20. The processaccording to claim 1, wherein the first step of the impregnation stageis concurrent and the second step is countercurrent.
 21. The processaccording to claim 1, wherein the impregnation stage is merelycountercurrent.
 22. The process according to claim 1, wherein in theimpregnation stage the effective alkali of the impregnation liquor doesnot exceed 1.5 mol/l NaOH.
 23. The process according to claim 1, whereinafter the impregnation stage no energy is introduced to the process fromthe outside to elevate the temperature.
 24. The process according toclaim 1, wherein the majority of the alkali to be added to the rawmaterial is added to the steps of the impregnation.
 25. The processaccording to any claim 1, wherein the cooking stage comprises both aconcurrent and a countercurrent zone, whose alkali concentrations andtemperatures can be regulated.
 26. The process according to claim 1,wherein the dynamic liquid-to-wood ratio of the cooking stage is about2-6 m³/Odt wood.
 27. The process according to claim 1, whereinanthraquinone is added to the impregnation stage and/or to the cookingstage.
 28. The process according to claim 1, wherein the cookingreactions are terminated in the digester by displacing the cookingliquor or by diluting using filtrate from a wash plant, whereby the blowis carried out at a temperature below 100° C.
 29. The process accordingto claim 1, wherein the pulp is blown out at a temperature essentiallyhigher than 100° C., optionally to a blow tank provided with heatrecovery.
 30. The process according to claim 28, wherein the pulp isblown out through a mechanically defibering stage to the blow tank. 31.The process according to claim 1, wherein the cooking process iscontinuous.
 32. The process according to claim 31, wherein thecontinuous cooking process is with one or two vessels.
 33. The processaccording to claim 32, wherein middle pressure or low pressure steam isintroduced to the steam/liquid phase digester for heating thereof. 34.The process according to claim 32, wherein the impregnation takes placein a separate impregnation vessel or in the upper part of the digesterbefore the cooking stage.
 35. The process according to claim 1, whereinthe impregnation and cooking stages are carried out in a batch cook. 36.The process according to claim 35, wherein the process is a displacementbatch cook.
 37. The process according to claim 35, wherein theimpregnation stages are carried out in a separate impregnation vesseloutside the digester.
 38. The process according to claim 1, wherein thechips are heated with steam to remove gases prior to slurrying the chipswith the impregnation liquor.
 39. The process according to claim 1,wherein the fresh alkali addition introduced to the impregnation liquorconsists of green liquor or green liquor and other fresh alkali,especially of white liquor.
 40. The process according to claim 1,wherein water is removed in the form of steam.
 41. The process accordingto claim 1, wherein the amount of the inorganic and organic dry solidmatter dissolved in the impregnation liquor is between 20-35%.
 42. Asystem for carrying out the process according to claim 1, comprisingmeans for impregnating and cooking the raw material, means forcirculating the impregnation liquor, means for introducing alkaliaddition, wherein the apparatus comprises means outside the impregnationapparatus for concentrating the impregnation liquor by removing water inthe form of steam.
 43. A system according to claim 42, wherein theapparatus includes means for removing from the cooking process the steamor part of the steam generated in the said removal of water.